Semiconductor modules have been widely used in power conversion devices typically used in hybrid automobiles or electric automobiles. In such a semiconductor module constituting a control device for saving energy, a power semiconductor element is provided to control a high electric current. The usual power semiconductor element generates heat when controlling a high electric current, and the amount of generated heat increases as the size of the power conversion device is reduced and the output thereof is increased. Therefore, it is very important to ensure cooling in a semiconductor module provided with a plurality of power semiconductor elements.
Cooling apparatuses of a liquid cooling system have been used to increase the cooling efficiency of semiconductor modules. A variety of measures have been used to increase the cooling efficiency of the cooling apparatuses of a liquid cooling system, for example, by increasing the coolant flow rate, forming the heat radiating fins (cooling bodies) such that good heat transfer coefficient is obtained, and increasing thermal conductivity of the material constituting the fins.
However, when the flow rate of the coolant flowing to the cooling apparatus is increased or a complex fin shape is used that has good heat transfer coefficient, a load on the cooling pump for circulating the coolant is increased, for example, due to the increase in the pressure loss of the coolant inside the apparatus. In particular, in a cooling apparatus that cools a large number of power semiconductor elements by using a plurality of heat sinks, the increase in pressure loss becomes especially significant in a flow channel configuration in which a plurality of flow channels is connected in series. In order to reduce the pressure loss, it is ideal to use a configuration in which the cooling efficiency is increased at a small coolant flow rate. For this purpose, for example, thermal conductivity of the fin material may be modified, but using a fin material with a high thermal conductivity can increase the cost of the entire apparatus.
In order to reduce the pressure loss while maintaining the cooling performance, cooling apparatuses have been suggested (see Patent Documents 1 to 8) in which a coolant introducing flow channel for introducing the coolant and a coolant discharge flow channel for discharging the coolant are arranged parallel to each other, and a plurality of heat sinks is disposed therebetween in the coolant circulation direction that is substantially perpendicular thereto. In this case, the flows of the coolant between the fins constituting the heat sinks are parallel to each other, the cooling performance can be increased, and also the pressure loss of the coolant in the flow channels can be reduced (see Patent Document 5).
Further, Patent Document 3 describes a cooling apparatus of a liquid cooling system in which the flow channels (header water channels 11a, 11b) for introducing and discharging the liquid coolant are disposed at the same side surface of a module, and the flow channels are disposed in the direction orthogonal to the fins, without variations in the cross-section area thereof (see FIG. 1). As a result, the pressure loss generated in the liquid coolant can be greatly reduced.
Further, Patent Document 6 describes a cooling apparatus of a liquid cooling system in which the entire rear wall of the casing constituting a liquid coolant inflow section is gradually inclined forward from the right wall side to the left wall side and the cross-section area of the flow channel in the inlet header section decreases from the liquid coolant inlet side to the left wall side. As a result, it is possible to improve uniformity of flow velocity distribution in the entire flow channel of the parallel flow channel portion of the casting, that is, the flow velocity distribution in the lateral direction of the parallel flow channel portion.    Patent Document 1: Japanese Patent Application Publication No. 2001-35981 (see paragraph [0020] and FIG. 1)    Patent Document 2: Japanese Patent Application Publication No. 2007-12722 (see paragraph [0006] and FIG. 7)    Patent Document 3: Japanese Patent Application Publication No. 2008-205371 (see paragraph [0021] and FIG. 1)    Patent Document 4: Japanese Patent Application Publication No. 2008-251932 (see paragraphs [0037] and [0038] and FIG. 7)    Patent Document 5: Japanese Patent Application Publication No. 2006-80211 (see paragraph [0006] and FIG. 1)    Patent Document 6: Japanese Patent Application Publication No. 2009-231677 (see paragraphs [0024] and [0031] and FIG. 2)    Patent Document 7: Japanese Patent Application Publication No. 2006-295178 (see paragraphs [0017] to [0024] and FIG. 2)    Patent Document 8: Japanese Patent Application Publication No. 2010-203694 (see paragraph [0026] and FIG. 3)
However, with the conventional cooling technique, a drift distribution, that is, the drift of coolant flow inside the cooling device, occurs due to the shape of the heat sink of coolant flow channels, the arrangement of heat generating elements, or the shape of the coolant introducing port and discharge port. Since such drift distribution causes uneven cooling performance, it is difficult to obtain stable and uniform cooling performance in the conventional cooling apparatuses. Another problem is that the heat generation temperature of the semiconductor element disposed at a position directly opposite the coolant discharge port side rises significantly, thereby decreasing the element service life and damaging the element.
Further, if the cross-sectional area of the flow channel in the inlet header portion decreases in the extension direction, as in the cooling apparatuses disclosed in Patent Documents 6 and 7, the flow rate distribution tends to improve, but the problem of temperature increase in the vicinity of the coolant introducing port remains unsolved and the increase in pressure loss is easily induced if the flow velocity adjustment is performed by changing the shape of the introducing flow channel.
In the cooling apparatus of a liquid cooling system described in Patent Document 8, a plurality of flow channel groups that includes a plurality of flow channels and differ in path resistance is provided side by side in the lateral direction of the parallel flow channel portion in the parallel flow channel portion, and the uniformity of the flow velocity distribution in the lateral direction of the parallel flow channel portion can be improved, and the occurrence of regions where the cooling performance is reduced due to the decrease in the flow velocity can be prevented. However, stable cooling performance cannot be easily obtained due, for example, to the warping of the fin base occurring in the process of manufacturing the cooling apparatus.